SUNRPC: Remove the server 'authtab_lock' and just use RCU

Module removal is RCU safe by design, so we really have no need to
lock the 'authtab[]' array.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>